The present invention relates to a membrane-based module for non-cryogenic dehydration and separation of a gas.
It has been known to use a polymeric membrane to separate air into components. Various polymers have the property that they allow different gases to flow through, or permeate, the membrane, at different rates. A polymer used in air separation, for example, will pass oxygen and nitrogen at different rates. The gas that preferentially flows through the membrane wall is called the “permeate” gas, and the gas that tends not to flow through the membrane is called the “non-permeate” or “retentate” gas. The selectivity of the membrane is a measure of the degree to which the membrane allows one component, but not the other, to pass through.
A membrane-based gas separation system has the inherent advantage that the system does not require the transportation, storage, and handling of cryogenic liquids. Also, a membrane system requires relatively little energy. The membrane itself has no moving parts; the only moving part in the overall membrane system is usually the compressor which provides the gas to be fed to the membrane.
A gas separation membrane unit is typically provided in the form of a module containing a large number of small, hollow fibers made of the selected polymeric membrane material. The module is generally cylindrical, and terminates in a pair of tubesheets which anchor the hollow fibers. The tubesheets are impervious to gas. The fibers are mounted so as to extend through the tubesheets, so that gas flowing through the interior of the fibers (known in the art as the bore side) can effectively bypass the tubesheets. But gas flowing in the region external to the fibers (known as the shell side) cannot pass through the tubesheets.
In operation, a gas is introduced into a membrane module, the gas being directed to flow through the bore side of the fibers. One component of the gas permeates through the fiber walls, and emerges on the shell side of the fibers, while the other, non-permeate, component tends to flow straight through the bores of the fibers. The non-permeate component comprises a product stream that emerges from the bore sides of the fibers at the outlet end of the module.
An example of a membrane-based air separation system is given in U.S. Pat. No. 4,881,953, the disclosure of which is incorporated by reference herein.
Other examples of fiber membrane modules are given in U.S. Pat. Nos. 7,497,894, 7,517,388, 7,578,871, and 7,662,333, the disclosures of which are all hereby incorporated by reference.
A polymer membrane becomes degraded in the presence of liquid water or water vapor. Therefore, the air directed into the membrane must be substantially free of water. For this reason, it is common to provide some form of dehydration unit which treats the gas before it enters the gas separation module. Polymers have been developed which separate water vapor from a gas. An example of such a polymer is given in U.S. Pat. No. 7,294,174, the disclosure of which is incorporated by reference herein.
The compressed air supplied to a membrane module must also be free of particulates and oil vapor, such as the particles of oil, and the oil vapors, which leak from the compressor. Carbon beds are typically used to remove such particles of oil, and the oil vapor, from the air stream. But excessive humidity also degrades the performance of such carbon beds, which is another reason why the air supplied to the module must be relatively dry.
In addition to a dehydration module and a carbon bed, one may provide heaters, moisture traps, and/or filters between the compressor and the membrane unit, as needed.
When the gas separation unit is installed on an aircraft, or in some other environment where space is limited, it becomes inconvenient or impractical to provide a dehydration membrane unit and a carbon bed, in addition to a gas separation module.
The present invention therefore provides an integrated module which combines the functions of a dehydration module and a gas-separation module, and which also includes an absorbent. The present invention therefore reduces the space required for such systems, and therefore makes it feasible to operate a membrane-based gas-separation system on board an aircraft, or in other limited-space environments.